A nude mouse model for human bone formation in unloaded conditions.

We describe an experimental model for human bone formation in unloaded conditions. Bone formation has been assessed by implanting in vivo human bone marrow stromal cells (BMSC) on porous hydroxyapatite (HA) bioceramics subcutaneously in nude mice. In this system, human bone formation and remodeling occurs and can be studied in unloaded conditions, i.e., with no influence of muscle tension. Using this model system, we have been also studying the effects of dexamethasone (Dex) in combination with fibroblast growth factor-2 (FGF-2) on the osteogenic potential of human BMSC. A colony-forming unit-fibroblastoid (CFU-F) formed in clonal conditions were significantly larger when Dex/FGF-2 was present in the culture medium. The cell proliferation rate was also increased by the combination Dex/FGF-2 at a higher extent than Dex or FGF-2 alone. BMSC expanded with Dex/FGF-2 displayed alkaline phosphatase levels lower (56%) than Dex expanded cells, but significantly higher than FGF-2 expanded cells. Our results suggest that Dex/FGF-2 expanded BMSC are able to form more bone than BMSC expanded in the presence of FGF-2 alone.